See attached link. Having recently experienced the same, I researched it. Evidently it is not as uncommon as most would think. I had radiation 3 years ago for PCa with extensive coverage to the SV. That may be the cause in my case. Generally speaking, the cause is most often benign.
/onlinelibrary.wiley.com/doi/full/10.1007/s12522-011-0087-4